Technical Abstract:
Fungal endophytes can influence host health, and more broadly, can instigate trophic cascades with effects scaling to the ecosystem level. Despite this, biotic mechanisms of endophyte community assembly are largely unknown. We used maize to investigate three potential habitat filters in endophyte community assembly: host defense compound production, production of such compounds by plant neighbors of the host, and presence of a seed endophyte. Maize (Zea mays) has been selectively bred to produce benzoxazinoids (BXs), compounds toxic to microbes and insects. BX-producing (BX+) and BX-nonproducing (BX–) genotypes were used in three field experiments to determine the influence of each factor on endophyte community assembly. In each study, fungi were isolated, identified and tested for tolerance to 2-benzoxazolinone (BOA), a toxic BX byproduct. Data on species diversity and the BOA tolerance of community members was used to characterize the functional diversity of communities. We found that the mean BOA tolerance
of communities and mean abundance of Fusarium species, including
agents of animal toxicosis, were significantly greater in BX+
than in BX–plants. For endophyte communities of BX– plants, proximity to BX+ plants had a significant effect on diversity. Communities from BX– plants grown in monoculture had higher species diversity than those from BX– plants grown in co-culture with BX+ plants. Results from the field and previous in vitro studies indicate that the seed endophyte, F. verticillioides, can detoxify BOA and facilitate colonization by species with low BOA tolerance in BX+ plants. Results from all three studies suggest that increased colonization by Fusarium may be an unintended consequence of selective breeding. Overall, results indicate
that both BXs and F. verticillioides are strong habitat filters in endophyte community assembly. Defense compounds are ubiquitous in
plants; we provide evidence for their role in structuring endophyte communities.